The famous Welch paper says Forty-eight of the 50 state health departments in the United States responded to a questionnaire about giardiasis in their jurisdictions. The agencies had reports of 34348 cases during 1991 and studied 80 outbreaks in the same period. Nineteen of these outbreaks were attributed to consumption of contaminated drinking water; only two outbreaks were reported among individuals identified as campers or backpackers. http://www.ncbi.nlm.nih.gov/pubmed/11995903
Following is my attempt to look at these numbers in a different, and what I think is a more telling, way.
The CDC estimates there are an upwards of 2.5 million cases of giardiasis annually in the U.S. http://www.giardiasis.org/
Using the 2-7% infection rate for developed countries would actually mean there are a minimum of 6,680,000 cases a year. But we'll take the lowest number of 2,500,000 cases, of which 34,348 were reported in 1991. 2,500,000 - 34,348 = 2,465,652 unreported cases. That means about 98.6% of cases go unreported. That figure supports what many physicians say, they usually don't report Giardia.
A disease outbreak is defined as "the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season." Thus, in 1991 there were only 80 outbreaks reported, or at least studied, out of 34,348 reported cases.
Now by my reasoning there would be a lot more reported outbreaks around large numbers of people sharing the same food and water (swimming pools, day care centers, rest homes etc,) if so the ratio of outbreaks/vs individual cases would skew towards the under reporting of individual backpacker Giardia, regardless of source. But we'll go with the low numbers again. Out of 80 actual outbreaks total, 2 were reported from contaminated drinking water from campers and backpackers. That's 2.5%.
If 2.5% of national Giardia cases are caused by camper/backpacker drinking water (2.5% times 2.5 million) that's 62,500 cases a year, minimum, of campers/backpackers contracting Giardia from contaminated water, extrapolating from the official numbers.
Now I grant there are some fuzzy numbers in there, but I think they are a lot more representative of the truth than this quote, based on the same fuzzy base numbers and used over and over again in literature attempting to promote the "myth" of getting giardia from backcountry drinking water: Nineteen of these outbreaks were attributed to consumption of contaminated drinking water; only two outbreaks were reported among individuals identified as campers or backpackers. That number of 62,500+ sounds a lot different than 2.
Check my numbers. If I've made any errors point them out and I'll make the changes.